Navigation processor, a system comprising such a navigation processor and a method of controlling an underwater system by such a navigation processor

ABSTRACT

A navigation processor is interfaced to an acoustic processor and to DGPS surface positioning equipment on a vessel. The navigation processor receives (a) position data from the DGPS surface positioning equipment, (b) data from a gyrocompass on an underwater system, (c) depth data of the underwater system, (d) velocity data of the underwater system based on data from a Doppler log unit on board the underwater system, and (e) range and bearing data of the underwater system from the acoustic processor, in order to calculate coordinates of the underwater system.

The present application is a continuation of U.S. Ser. No. 10/875,248.

BACKGROUND OF THE INVENTION

WO.99/61307 discloses an apparatus for deploying an object to anunderwater target position, the apparatus being provided with a beaconto transmit acoustic rays and a plurality of thrusters to controlpositioning of the apparatus with respect to the underwater targetposition.

The prior art apparatus is used for deploying and/or recovering loads upto 1000 tons or more on the seabed at great depths, for instance, up to3,000 meter or more. During deployment, the apparatus is controlled bycontrolling equipment on board of a vessel floating on the sea surface.The controlling equipment needs to know the exact location of theapparatus as accurate as possible. To that end, the beacon on board ofthe apparatus transmits acoustic rays through the sea water to thevessel. An appropriate acoustic receiver receives these acoustic raysand converts them into electrical signals used to calculate the positionof the apparatus with respect to the vessel.

However, it is found that with increasing depth of the apparatus belowthe sea water the accuracy of the location measurement decreases due tobending of the acoustic rays in the sea water.

SUMMARY OF THE INVENTION

The object of the invention is therefore to further enhance the accuracyof the location measurement of such an apparatus during use in sea wateror any other fluid. Moreover, such location measurement is neededon-line (real-time).

To obtain this object, the invention is directed to an arrangement asdefined in claim 1. Embodiments are defined in depending claims, whereasalternative aspects of the invention are claimed in other independentclaims.

The underwater apparatus, that is no part of the claims of thiscontinuation application, may be provided with a sound velocity meter tomeasure velocity of sound in a fluid surrounding said apparatus. Thus,the velocity of sound at a certain location in the fluid can becontinuously measured and used to update a sound velocity profile, i.e.,data as to the sound velocity as a function of depth in the fluid. Fromthese data, local bending of the acoustic rays can be determined on-line(real-time). So far, such on-line determination has not been possible.This allows corrections of location measurements in real-time.

The system as described comprises a processing arrangement, and a soundvelocity meter to measure velocity of sound in a fluid surrounding anunderwater apparatus, the processing arrangement being provided with anacoustic receiver to receive the acoustic rays, the processingarrangement is arranged to use data derived from the acoustic rays in acalculation to determine the position of the underwater apparatus. Theprocessing arrangement is arranged to receive online sound velocitymeter data from the sound velocity meter to determine a sound velocityprofile in the fluid and to calculate from the sound velocity profilebending of the acoustic rays transmitted by the apparatus through thefluid and to use this in the calculation to determine the position ofthe apparatus in real-time.

The processing arrangement constantly receives sound velocity data anddetermines a sound velocity profile comprising sound velocity data fromthe water surface to the depth of the apparatus. The processingarrangement uses these data to determine acoustic ray bending as afunction of the depth in the water and thus to correct any positioncalculation of the apparatus.

The sound velocity meter is arranged to be provided just below the watersurface to provide actual data regarding any ray bending in the watersurface layers and thus allows further correction of any positioncalculation of the underwater apparatus.

The acoustic array is arranged to be attached to a hull of a vessel. Theacoustic array is provided with a distinct gyrocompass measuring heave,roll and pitch. Output data from this gyrocompass are used to furtherincrease accuracy of the position measurement of the underwaterapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Below, the invention will be explained in detail with reference beingmade to the drawings. The drawings are only intended to illustrate theinvention and not to limit its scope which is only defined by theappended claims.

FIG. 1 shows a schematic overview of a FPSO (floating, production,storage and offloading system) dedicated to offshore petrochemicalrecoveries.

FIG. 2 shows a crane vessel according to the prior art and displaying aload rigged to the crane block with relatively long wire ropes wherebyit is possible to see that the control of the load is virtuallyimpossible at great depth.

FIG. 3 shows a crane vessel and an underwater system for deployingand/or recovering a load to and/or from the seabed according to theprior art.

FIG. 4 shows a detailed overview of a possible embodiment of theunderwater system.

FIG. 4 a shows a detailed overview of one of the rotatable thrusters.

FIG. 5 shows the underwater system viewed from above.

FIGS. 6 a and 6 b schematically show the underside of the main modulewith some detectors.

FIG. 7 a shows a schematic block diagram of the electronic equipment onboard of the vessel.

FIG. 7 b shows a schematic block diagram of the electronic equipmentrelated to an acoustic array and related to the underwater system.

FIG. 8 shows the definition of three different coordinate systems usedduring driving the underwater system to its target position.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, the layout presents a FPSO 1 with swivelproduction stack 11 from which risers 2 depart, said risers connectingto their riser bases 3 at the seabed 4. During production lifetime, itis paramount for the FPSO 1 to remain within an allowable dynamicexcursion range and therefor the FPSO 1 is moored to the seabed 4 bymeans of mooring legs 5 which are held by anchors 6, or alternatively bypiles.

Exploitation of oil or gas by means of a production vessel 1, requiresthat several relatively heavy objects be positioned at the seabed 4 witha high accuracy.

To secure an appropriate and safe anchoring by means of the mooring legs5, it is required that these mooring legs 5 have approximately the samelength. In practice for this application anchors can be used with aweight of 50 ton and more, which are placed at the seabed 4 with anaccuracy to within several meters. Moreover not only is the anchor 6itself very heavy, but the mooring leg attached to the anchor 6 has aweight that equals several times the weight of the anchor 6 itself.

Also for other objects like the “templates”, “gravity riser bases”,“production manifolds” etceteras applies that these objects have to beput on the seabed 4 with relatively high accuracy.

The objects that are shown in FIG. 1 that are required for exploitingthe oil and gas at sea and that have to be put on a seabed, are not onlyvery heavy, but very expensive as well.

FIG. 2 shows a vessel 20, according to the prior art, having hoistingmeans thereon, like a crane 21. The crane 21 is provided with a hoistingwire 22, by means of which an object or a load 4 can be put on theseabed 5. In order to position the load 23 it is necessary to move thesurface support together with the crane 21.

The result will be that, at one given time, the load 23 inertia will beovercome but due to the load 23 acceleration, an uncontrollablesituation will occur, whereby the target area will be overshot. Becauseof the fact that the hoisting wire 22 and the load 4 are susceptible toinfluences like the sea current, the load 23 will not move straightdownward, when the hoisting wire 22 is being lowered. Also the heave,roll and pitch of the vessel 20 will have a negative influence on theaccuracy that can be achieved.

FIG. 3 shows a crane vessel 40 provided with an underwater apparatus orsystem 50 for deploying a load 43 on the seabed 4. The vessel 40comprises first hoist means, for example a winch 41, provided with afirst hoist wire 42. By means of this hoist wire 42 the load 43, forinstance a template can be deployed and placed at the bottom of the sea.

As mentioned above, the exploitation of oil and gas fields using afloating production platform requires that several heavy objects must beplaced at the seabed 4, moreover, these objects have to be placed on theseabed 4 with a very high accuracy. Because of the fact that nowadaysthe exploitation has to be done at increasing depths up to 3000 m andmore, achieving the required accuracy is getting harder.

FIG. 4 shows a detailed overview of a possible embodiment of the system50 for deploying a load 43 on the seabed 4. FIG. 5 shows the systemaccording to FIG. 4, from above.

The system 50 comprises the main-module 51, the counter-module 52 and anarm 53. The arm 53 can be detached from the main-module 51. That meansthat the main-module 51 can also be used separately, as a modularsystem. The arm 53 is provided with a recess 54. On opposite sides ofthis recess 54 two jacks 57, 58 are provided, at least one of which canbe moved relative to the other. In between the end surfaces of thesejacks 57, 58 an object, such as a crane-block of load 43, can beclamped. In order to improve the contact between the jacks 57, 58 andthe object, the respective ends of the jacks are accommodated withclamping shoes lined with a friction element, from a high frictionmaterial such as dedicated rubber.

In use, the thrusters 56(i) can be used to position the system 50relative to a target area on the seabed 4. The thrusters 56(i) can beactuated from a first position mainly inside the system 50, to aposition in which the thrusters projects out of the system 50. The twoupper thrusters 56(2), 56(3) are rotatable with respect to theunderwater system 50. They are, for instance, installed on respectiverotary actuators 65(1), 65(2). The purpose thereof will be explainedlater. Thruster 56(2) has been shown on an enlarged scale in FIG. 4 a.

In FIG. 5 it is shown that there are two positions 61, 62 on top of themain-module 51 to connect the main module to the second lifting wire 45and/or to the umbilical 46. When the main-module 51 is used separatelyposition 61 can be used. The main-module 61 will be balanced when themodule 61 is deployed, both in the air and underwater.

When the system 50 is used, the connection between the vessel 40 and thesystem 50 will be fixed in position 62 in order to keep the system inbalance, both in the air and underwater. To improve the balance of thesystem, an auxiliary counterweight 55 can be secured to the system 50.

In use, the apparatus 50 will not have any buoyancy. In order to improvethe movability of the system under water, the arm 53 is provided withholes 59, in order to avoid structural damage due to an increasingpressure while being lowered and to ensure quick drainage during therecovery phase.

As mentioned above, it is advantageous when the counter-module 52 can bemoved relative to the main-module 51. This can be accomplished by usingjacks 64 a.

The module 51 comprises an outer frame and an inner frame (both notshown). The inner frame preferably is cylinder-shaped. By connecting theouter frame to the inner frame, a very strong construction can beaccomplished. The strength of the construction is necessary in order toavoid premature fatigue in the system.

The module 51 is, for instance, partly made of high-tensile steel andthereby designed to be used as integral part of either the first 42 orsecond hoist wire 45. This means that the top side of the module 51 willbe connected to a first part of the hoist wire 45, and that theunderside of the module 51 will be connected to a second part of thehoist wire 45, or the underside of the module 51 will be attacheddirectly to the load. In this way the load on the hoist wire will betransferred through the module 51.

As mentioned before, the module 51 is provided with a thruster drive 270for converting electrical power, delivered through the umbilical 46,into hydraulic power. This thruster drive 270 may comprise motors, apump, a manifold and a hydraulic reservoir. Such converting means areknown to persons skilled in the art and need no further explanationhere. In order to communicate relevant data as to its position, bothabsolute and relative to other objects, to the control system and/or anoperator on board of the vessel 40, the module 51 further comprisessensor means and control means that will be explained in detail below.The module 51 is equipped with a sensor junction box. Moreover, themodule 51 comprises light-sources 87, a gyrocompass 256 including heave,roll and pitch sensors, a pan and tilt color camera 97, a USBL responder255 including a digiquartz depth sensor 253, a sound velocity meter 258,and a sonardyne mini Rovnav 264. At the underside of the module 51 aremounted on several platforms light sources 94, a pan and S.I.T. camera93, an altimeter 262, a Doppler log unit 266, and a dual head scanningsonar 260. They are installed there to have only clear sea water belowthem, in use. They are schematically shown in FIGS. 6 a and 6 b. It isto be understood that they may be located elsewhere, e.g., at theunderside of module 52. Moreover, load cells 268 are part of the system51. All these components are schematically indicated in FIG. 7 b.

As mentioned above, the use of high resolution sonar equipment 260together with a distance log, measured by Doppler log unit 266, isimportant to achieve the required accuracy, once the load has reachedits intended depth. The sonar equipment 260 will be used to determinethe position with respect to at least one object positioned at theseabed. Using the distance log, it will then be possible to dissociatethe positioning activities from the surface support, as well as from anyother acoustic transponder devices such as LBL (Long Base Line) arrays(or other, e.g., USBL), while accuracy in the order of centimeters willbe achieved within a large radius.

FIG. 7 a shows the electronic equipment 200 installed on the vessel 40,whereas FIG. 7 b shows deployable acoustic array 250 with velocity meter248 and a gyro compass 252. FIG. 7 b also shows underwater electronicequipment 249 installed on the underwater system 50.

The equipment shown in FIG. 7 a comprises four processors: a navigationprocessor 202, acoustic processor 224, a sonar control processor 236,and a thruster control processor 240. The navigation processor 202 isinterfaced to the other three processors 224, 236, 240 for mutualcommunications and complementarity.

The navigation processor 202 is also interfaced to a surface positioningequipment DGPS (Differential Global Positioning System) 204, a vesselgyrocompass 206, four display units 208, 210, 212, 214, a printer unit218, a keyboard 220, a mouse 222, and a fiber optic (de)multiplexer unit244. If necessary, a video splitter 216 may be provided to transmit oneSVGA signal output of the navigation processor 202 to two or moredisplay units. In FIG. 7 a, display units 212, 214 are connected to thenavigation processor 202 via video splitter 216.

The fiber optic (de)multiplexer unit 244 is also connected to theacoustic processor 224, the sonar control processor 236, and thethruster control processor 240.

The acoustic processor 224 is connected to a command and control unit226 which is connected to a keyboard 230, a mouse 232 and a display unit228, all together forming a USBL surface unit 234.

The acoustic processor 224 is connected to deployable acoustic array 250with motion sensor unit 252 and velocity meter 248. In use, the acousticarray 250 is, preferably, mounted 2.5 meters below the keel of vessel40.

The fiber optic (de)multiplexer unit 244 is connected to a further fiberoptic (de)multiplexer 246 installed on the underwater system 50. Anoptical fiber interconnecting both fiber optic (de)multiplexers 244, 246is preferably accommodated in umbilical 46 (FIG. 3).

The sonar control processor 236 is connected to a display unit 238. Thethruster control processor 240 is connected to a display unit 242.

The underwater equipment 249 is shown in FIG. 7 b in the form of a blockdiagram. The USBL responder 255 with digiquartz depth sensor 253, agyrocompass with motion sensors 256, (removable) sound velocity meter258, a dual head scanning sonar 260, altimeter 262, sonardyne miniRovnav 264, Doppler log 266, load cells 268, and thruster drive control270 are all connected to the fiber optic (de)multiplexer 246.

Moreover, FIG. 7 b shows two beacons 272, 274 that can be installed onthe seabed or on the load to be deployed (or on other structures alreadyon the seabed). These beacons 272, 274 can, e.g., be interrogated bymeans of the sonardyne mini Rovnav 264 (or equivalent equipment) totransmit acoustic signals back to the system 50 that can be used by thesystem 50 itself to determine and measure distances and orientationsrelative to these beacons. Such an acoustic telemetry link results invery high precision relative position measurements. The number of suchbeacons is not limited to the two shown in FIG. 7 b.

Functionality

The functions of the components shown in FIGS. 7 a and 7 b are thefollowing.

The navigation processor 202 is collecting the surface positioningequipment data (DGPS receivers, DGPS corrections, vessel's gyrocompassand vessel's motion sensors 204 and 206), in order to calculate anddisplay the vessel's attitude and its fixed offsets.

Via the fiber optic (de)multiplexers 244 and 246, the navigationprocessor 202 sends different settings to the navigation instruments ofthe system 50, i.e., Doppler log 266, altimeter 262, and gyrocompass andmotion sensors 256. After setting up, it receives the data from thoseinstruments, as well as, via the acoustic processor 224, therange/bearing and depth data of the system 50 to calculate and todisplay the attitudes and absolute coordinates of the system 50.

An integrated software in the navigation processor 202 has beendeveloped, including a dynamic positioning controller software able towork in manual or automode to decide the intended heading of the system50 and to select between many way points and to carry out the intendedpositioning. Moreover, the operator on board of the vessel can inputoffsets to the selected way point, the offsets being input with XYcoordinates relative to the heading of the system 50. There is anotherpossibility to select several other types of sub-sea positioning devicesvia an arrangement of specifically designed windows on the screens(electronic pages) of the display units 208-214, to stabilize and filterthe position. To ensure that the operator has as many tools as possibleto get the optimal result, there is an other part in the softwareshowing different status of the sub-sea instruments in use for thecalculation of the position of the system 50 on-line (real-time).

Embarked gyrocompass 256 including heave, roll and pitch sensors 88 onboard of the system 50 provides data as to the exact attitudes of boththe system 50 and the load 43 to be installed on the sea bed. At thesurface of the sea, in a control van, operators are able to check thoseattitudes on-line (real-time), during descent but also once the load 43is laying on the sea bed for final verification.

The vessel gyrocompass 206, as well as the gyrocompass with motionsensors 252 installed on the acoustic array 250 that could be used forthe same functions, is transmitting the vessel's heading to thenavigation processor 202. The navigation processor 202 will use thisvessel's heading to calculate different offsets.

The display units 208, 210, 212, and 214, respectively, are arranged todisplay navigation settings, a view of the sea bed, a view of thesurface, in the control van for the operators and another one on thevessel bridge for the marine department operators.

The USBL command and control unit 226 consists of a personal computerproviding control and configuration of the system and displaying theman-machine-interface for operator control.

The acoustic processor 224, preferably, consists of one VME rack whichperforms correlation process on received signals, corrections tobathy-celerimetry and vessel's attitude. Moreover, it calculatescoordinates of any beacon used. The acoustic processor 224 is linked tothe navigation processor 202 through Eternet.

The acoustic array 250 includes means for transmission and reception.The acoustic array 250 can be used as a transducer to acousticallycommunicate with one or more beacons. Such a transducer mode isadvantageous when the umbilical 46 fails and is unable to transmitinterrogation signals down to the system 50. Then, acousticinterrogation signals can be transmitted down by the transducer directlythrough the sea water. In all other cases, the acoustic array 250 willbe used in a reception mode. Reception is done with two orthogonalreception bases which measure distances and bearing angles of beaconsrelative to the acoustic array 250. Each reception base includes twotransducers. Each received signal is amplified, filtered and transferredto the acoustic processor 224 for digital signal processing.

The sound velocity meter 248 installed on the acoustic array 250 isupdating in real-time the critical and unsettled sound velocity profilesituated just underneath the vessel 40. This is of great importancesince turbulences of the sea water appear to be very heavy in theselayers just underneath the vessel 40.

The gyrocompass 252 is preferably used as motion sensor unittransmitting the acoustic array attitude to the acoustic processor 224in order to rectify data as to the position of the system 50 sub-sea.

In a preferred embodiment, the beacon 255 is working in a responder modeand has the following characteristics:

-   -   the triggering interrogation signal generated by the acoustic        processor 224 is not acoustic but electrical and is transmitted        to the beacon 255 through the cable link between the vessel 40        and the system 50;    -   interrogation frequencies are remotely controlled by an operator        through the man-machine-interface.

As indicated above, the beacon 255 can also be used in a transpondermode. Then, the beacon 255 is triggered by a surface acoustic signaltransmitted by the acoustic array 250 and then delivers acoustic replysignals to the acoustic array 250 through a coded acoustic signal.

The digiquartz depth sensor 253 included in the beacon 255 allowstransmitting very accurate depth data of the system 50 to the acousticprocessor 224. The acoustic processor 224 uses these data to improve thecalculation of the sub-sea positioning of the system 50 and its load 43.

The sound velocity meter 258, mounted on the underwater system 50, istransmitting data as to the velocity of sound in sea water at the depthof the underwater system 50 to the acoustic processor 224 during descentand recovery. The sound velocity data is used to update calculated soundvelocity profiles in the sea water as a function of depth in real-timeand to calculate acoustic ray bending from these profiles as function ofdepth in the sea water and thus to correct calculations of the sub-seaposition of the system 50.

The dual head scanning sonar 260 is used to measure ranges and bearingsof the system 50 to any man-made or natural target on the seabed and tooutput corresponding data as digital values to the navigation processor202. The positions of such man-made or natural targets can either bepredefined or the navigation system can allocate coordinates to each ofthe selected objects. After the objects have been given coordinates,they can be used as navigation references in a local coordinate system.This results in an accuracy of 0.1 meter in relative coordinates.

The altimeter 262 mounted on the system 50 is measuring the verticaldistance of the underwater system 50 to the seabed and transmits outputmeasuring data to the acoustic processor 224.

The Doppler log unit 266 provides data as to the value and direction ofthe sea water current at the depth of the underwater system 50. Thesedata are used in two ways.

First of all, the data received from the Doppler log unit 266 and thegyrocompass with motion sensor 256 is used by the acoustic processor 224to smooth on-line (real-time) the random noise related to using USBL. Toobtain such a smoothing a filter is used, e.g., a Kalman filter, aSalomonsen filter, a Salomonsen light filter, or any other suitablefilter in the main processor unit 224. Such filters are known to personsskilled in the art. A brief summary can be found in appendix A.

Secondly, the output data of the Doppler log unit 266 regarding currentstrength, current direction, together with data regarding present andintended heading of the underwater system 50 are transmitted to thethruster control processor 240 via the navigation processor 202. Basedon the intended direction the thruster drive control 270 will beautomatically controlled. Manual control may also be provided for.

In a very advantageous embodiment the Doppler log unit 266 (or any othersuitable sensor) is used to measure temperature and/or salinity of thesea water surrounding the system 50. Data as to local temperature and/orsalinity is transmitted to the navigation processor 202 that calculatesand updates temperature and/or salinity profiles as a function of depthin the sea water. These data are also used to determine acoustic raybending through the sea water and, thus, to correct calculations of theposition of the system 50.

The sonardyne mini Rovnav 264 is optional and may be used to providerelative position of the system 50 to local beacons on the seabed asexplained above. For instance, a Long Base Line (LBL) array may alreadybe installed on the seabed and used for that purpose.

The thruster drive control 270 is used to drive the thrusters 56(i) inorder to drive the underwater system 50 to the desired position as willbe explained in detail below.

In FIG. 7 a, four different processors 202, 224, 236 and 240 are shownto carry out the functionality of the system according to the invention.However, it is to be understood that the functionality of the systemcan, alternatively, be carried out by any other suitable number ofcooperating processors, including one main frame computer, either inparallel or master slave arrangement. Even remotely located processorsmay be used. There may be provided a processor on board of theunderwater system 50 for performing some of the functions.

The processors may have not shown memory components including harddisks, Read Only Memory's (ROM), Electrically Erasable Programmable ReadOnly Memory's (EEPROM) and Random Access Memory's (RAM), etc. Not all ofthese memory types need necessarily be provided.

Instead of or in addition to the keyboards 220, 230 and the mice 222,232 other input means known to persons skilled in the art, like touchscreens, may be provided too.

Any communication within the entire arrangement shown may be wireless.

In FIG. 5, the situation is shown that the two upper thrusters 56(2) and56(3) are directed in an other direction than the thrusters 56(1) and56(4). The thrusters 56(2), 56(3) are mounted on rotary actuators 65(1),65(2), which allow the thrusters 56(2), 56(3) to be vectored by turningthem up to 360°. Preferably, the thrusters 56(2), 56(3) can beindependently controlled such that they may be directed each to adifferent direction.

To allow the thruster control processor 240 to accurately position theunderwater system 50, a common coordinate system must be establishedbetween the navigation processor 202 and the thruster control processor240. First of all, there is a standard coordinate system used by thenavigation processor 202. However, two other coordinate referencesystems are preferably established for the underwater system 50.

FIG. 8 shows the three different coordinate systems. The coordinatesystem related to the navigation processor 202 is indicated with“navigation grid”. This coordinate system uses this “navigation grid”direction and its normal.

The thrusters 56(2), 56(3) are controlled to provide a driving force ina direction termed “thruster mean direction”. This direction togetherwith its normal defines the second coordinate system.

The third coordinate system is defined relative to the “systemdirection” which is defined as the direction perpendicular to a lineinterconnecting the thrusters 56(1), 56(4).

Now, an error in the path followed by the underwater system 50 can bedefined in terms of an error vector that can be split into one componentparallel to the thruster mean direction termed the “mean error” and acomponent normal to the thruster mean direction termed “normal meanerror”. Appropriate sensors on the underwater system 50 will provide thenavigation processor 202 with the thruster mean direction and systemdirection. From these data the navigation processor 202 will create agrid as shown in FIG. 8.

The error is defined as the desired position DP minus the systemposition TP such that a vector RΦ_(EN) is generated relative to thenavigation grid reference, i.e.:DP−TP=RΦ _(EN)

Moreover:

Φ_(TN) is the system orientation minus the navigation grid orientation,

Φ_(MT) is the mean thruster orientation minus the system orientation.

Then:DP−TP=RΦ _(EM),Φ_(EM)=Φ_(EN)−(Φ_(TN)+Φ_(MT))Now RΦ_(EM) is known, the mean and the normal to the mean errors can becalculated.

The two thrusters 56(1) and 56(4) are used to counteract the twistingforces applied by the lifting cable 42, equipment drag and therotational moment induced by the vectoring of positioning control. Acontrol loop for the orientation requires that the navigation processor202 is provided with the actual system orientation and the desiredsystem orientation. The actual system orientation is measured by thegyrocompass 256. The desired orientation is manually input by anoperator. From these two orientations the control loop in the navigationprocessor 202 computes an angular distance between the requiredorientation and the actual orientation as well as the direction ofrotation required to move the system 50 accordingly. A simple controlloop controlled by the thruster control processor 240 then adjusts thepower to the thrusters 56(1) and 56(4) to rotate the system 50appropriately.

On power up of the system 50, both thrusters 56(2) and 56(3) will be,preferably, orientated such that the thruster mean direction is directedparallel to the system direction. Then, the thrusters 56(2), 56(3) willbe given a small vector angle deviation from the system direction toassist in positioning the system 50 in two planes. The size of thisvector is, preferably, manually adjustable and may be needed to beconfigured for each different job in dependence on actual seaconditions. Once the thrusters 56(2) and 56(3) have been centered andvectored, a positioning loop can take over control of the system 50.

The positioning loop comprises two more phases.

In the first next phase, which is executed while the system 50 is stillnear the sea surface, the sea current direction will be measured by theDoppler log unit 266. The sea current direction will be transmitted tothe navigation processor 202. Using this direction, the thruster controlprocessor 240 receiving proper commands from the navigation processor202 will drive the rotary actuators 65(1), 65(2) such that the thrustermean direction substantially opposes the sea current direction. Duringthis rotation of the rotary actuators 65(1), 65(2) none of thrusters56(i) is powered. The system direction will be measured by the fiberoptic gyrocompass 256. The depth is constantly measured by thedigiquartz depth sensor 253 and the altitude by the altimeter 262. Themean and normal to the mean errors as calculated in accordance with theequations above will then be used by the positioning loop to apply powerto the thrusters 56(2) and 56(3) to drive the system 50 to the desiredlocation.

During driving the system 50 with load 43 to the desired coordinates bymeans of thrusters 56(2), 56(3) the thrusters 56(1), 56(4) are used tocounteract any rotation of the system 50 with its load 43. This providesfor better control since, especially for heavy loads, rotation movementsmay result in other undesired movements of the load, which may bedifficult to control. When the system 50 with its load is on the desiredcoordinates the load together with the system 50 is lowered by means ofthe hoisting wire 42. During descending the load 43, the load 43 isconstantly controlled by system 50 to keep it on the desired locationwithout any rotation.

In a next phase, the system 50 is for instance approximately 200 m orless from the seabed 4. Then, the Doppler log unit 266 goes into abottom track mode. This changes the operation into a more accurate andfast responding mode for the final approach of the target location onthe seabed 4. Now, the Doppler log unit 266 and the gyrocompass withmotion sensors 256 are used to filter the random noise of the USBL. Oncefiltered, a good read out of the navigation data including an accuratevelocity of the system 50 will make the position control loop bothextremely rapid and stable. A very fine tuned control loop results inwhich control up to some centimeters movement is achieved. Now, thesonar unit 260 and the Doppler log unit 266 are used to provideinformation regarding the surroundings of the target point such that theload 43 can be positioned on the right coordinates and in the rightorientation. Then, a rotation, if necessary, may be applied to the load43 by thrusters 56(1), 56(4) as controlled by thruster control processor240.

Two control loops are provided for the thrusters 56(2), 56(3): a meanerror control loop and a further control loop to reduce the normal meanerror.

The mean error control loop will adjust the power equally to boththrusters 56(2), 56(3) so as to reduce the mean error. As the system 50reaches the target coordinates the driving power to the thrusters 56(2),56(3) will be reduced to such a level that the system 50 is able tomaintain its position in the sea current. In other words, initially, thedriving power was set at a level that was proportional to the meanerror. However, as the system 50 moves closer to the target coordinatesthe control loop will slowly reduce the driving power applied to thethrusters 56(2), 56(3). As the system 50 reaches the target coordinatesan equilibrium will be reached where the driving power to the thrusters56(2), 56(3) counteracts the strength of the sea current. The mean errorcontrol loop provides equal power with equal sign to both thrusters56(2), 56(3).

A further control loop is applied to reduce the normal mean error. Thisfurther control loop adjusts the individual power applied to thethrusters 56(2), 56(3) such that a movement perpendicular to the seacurrent is generated. The further control loop applies equal power ofopposite sign to both thrusters 56(2), 56(3) to this effect. The powerapplied to the thrusters 56(2), 56(3) in order to reduce the normal meanerror, preferably, reduces linearly to zero as the system 50 moves tothe target coordinates. At the point where the normal to the mean errorreaches zero and assuming that the sea current direction has notchanged, the system 50 will exactly be located above the target positionon the sea bed 4 and the thrusters 56(2), 56(3) are powered to keep thesystem 50 on the correct coordinates and to correct for the sea current.

If the sea current direction changes the control loops referred to abovewill be required to adjust the power applied to the thrusters andultimately to change the system direction. As the new current directionacts upon the system 50, the normal mean error will start to increase asthe system 50 is moved from the target coordinates. To overcome thiseffect, the size of the normal mean error will again be controlled toreduce to zero. The system direction is changed such that the seacurrent or natural drift of the system 50 is counteracted.

The direction of rotation of the rotary actuators 65(1), 65(2) will bedefined by the sign of the normal mean error. To reduce the timerequired to slew the rotary actuators 65(1), 65(2) to the requiredposition, an algorithm will be used by the thruster control processor240 to determine the shortest route to the required orientation.

It is envisaged that manual control by means of, for instance, ajoystick (not shown) connected to the navigation processor 202 is alsoarranged.

During the positioning of the system 50 a velocity control is also,preferably, applied. Preferably, the closer is the system 50 to thecoordinates of the target, the slower will be the velocity of the system50. For instance, when the distance between the system 50 and the targetis more than a predetermined first threshold value, the thrusters arecontrolled to provide the system 50 with a maximum velocity. Betweenthis first threshold value and a second threshold value of the distanceto the target coordinates, the second threshold value being lower thanthe first threshold value, a linearly decreasing velocity profile isused. Within a distance smaller than the second threshold value thesystem is kept on a velocity of substantially zero.

USBL Measurement

The USBL measurement principle is based on an accurate phase measurementbetween two transducers. In one embodiment, a combination of short baseline (SBL) and ultra short base line (USBL) is used which enables to usea large distance between transducers without any phase ambiguity. For anUSBL, the accuracy depends on the signal to noise ratio and the distancebetween the transducers (like in an interferometry method). Then, thetrade-off is for frequency which is limited by the range andhydrodynamic part in terms of dimensions.

Ambiguity is calculated by using an SBL measurement combined withcorrelation data processing. The signal-to-noise ratio is improved byuse of such correlation processing. The following expression defines thegeneral accuracy for a USBL:

$\sigma_{\theta} = {K\frac{\lambda}{L\sqrt{\frac{signal}{noise}}\cos\;\vartheta}}$where:σ_(θ): Angular standard deviationL: transducer distanceλ: wavelengthθ: bearing angle

The expression given above indicates that the accuracy is improved byincreasing the transducer distance L, i.e., by increasing the array.Moreover, a higher frequency results in a better accuracy. Hydrodynamicaspects and phase ambiguity reduce these parameters. Signal-to-noiseratio is increased by using correlation data processing.

To optimize range and accuracy, a frequency of 16 kHz is preferably usedfor phase meter measurements. A correlation process enables to increasethe distance range while keeping a narrow pulse length for multipathdiscrimination.

For ambiguity phase measurements, the system operates in SBL todetermine a range sector and in USBL within the sector to achieve thebest accuracy.

The range may be increased beyond 8000 m by using a rather lowfrequency.

Appendix A

Kalman Filter

The Kalman filter is probably the most well-known technique in theoffshore industry. It gives a fast filtering method based on comparisontowards predicted values, which are calculated on basis of the latesthistory. We will not go into details about Kalman filtering, but referto, e.g., “Kalman Filtering—Theory and Practice”, by M. S. Grewal and A.P. Andrews Prentice Hall (ISBN 0-13-211335-X).

The position track can be combined with the velocity data (Doppler log),each point will be improved on basis of the neighboring points, thedistance in time and the actual speed. The weight between Kalman valueand the velocity improved is decided by the Doppler efficiencycoefficient: higher values will take speed more into consideration.

Advantage: Disadvantage: It's fairly fast Rather ‘un-smooth’ result Canbe improved with speed Not the best combination of speed and positionSimple Filter

The Simple filter runs through all positions, and calculates a smoothcurve giving a minimum squared error, i.e. a kind of Least Square Fitline

Advantage: Disadvantage: It's fast No Doppler-log data is used Theresult is smooth Does not like curved tracksSalomonsen Filter

The Salomonsen filter, which is named after the Danish mathematicianHans Anton Salomonsen, Professor and phD at University of Aarhus, is ahighly integrated filter. It takes advantage of the short-term stabilityof the Doppler track and combines it with the long-term robustness ofthe position track.

Description

The filter is used in a situation where we have time tacked positiondata along a track as well as Doppler data. The Doppler Data are usuallyvery precise but do not give any information about the absolutepositions. On the other hand the position data are absolute positionsbut they are usually not very precise.

The filter combines the two sets of data to produce a precise track withabsolute positions. This is done as follows.

1. The Doppler data are used to construct the shape of the track, i.e. atrack formed as a cubic spine.

2. Beginning at the origin (0, 0) and with velocities as defined by theDoppler data.

3. Then the position data are used to position the track correctly. Thetrack is translated, rotated, and stretched/compressed linearly to fitthe position data as well as possible using least squares techniques.

4. It will mainly be a translation. However, the other modificationsserve to correct for possible systematic errors in the Doppler data.

The fact that the position data are used only to make the modificationsin 2 means that the position data are subject to considerable averaging.This reduces the uncertainty of the position measurements. Thus, ifthere are many position data the absolute position of the track shouldbe expected to be much more precise than each single positionmeasurement.

H.A. Salomonsen

Mathematical Description

The algorithm is divided into five steps:

Step 1:

Calculate accelerations for each point½hk+1(X1″+Xk+1″)=Xk+1′−Xk′Wherehk=tk−tk−1tk=timestamp for speed measurementXk′=speed measurement at tkXk″=calculated acceleration at tkStep 2:Calculate next position based on acceleration and speed, and previouscalculated position (based on previous speed measurements andaccelerations)Xk+1=Sqr(hk+1)/6(2Xk″+Xk+1′)+hk+1Xk′+XkWherexk=calculated position at tk (speed timestamp)Step 3:Calculate the positions at actual timestamps (using position of firstspeed measurements)X(t)=½hk+1{((hk+1)^2(t−tk)+⅓(tk+1−t)^3−⅓(hk+1)^3)Xk″+⅓(t−tk)^3 Xk+1″}WhereX(t)=position at time tStep 4:Add position of first speed measurements to calculated positionsStep 5:Move, rotate, stretch of compress calculated positions to best fit ofreal position line

Advantage: Disadvantage: It combines the best of Doppler It is slow dueto complex matrix and positions. Dependent on good Doppler-log Takes alldata in consideration The result is smoothSalomonsen Light

The light version of Salomonsen filter, which was first introduced inthe NaviBat On-line program, was invented to have a faster solutioncombining the better of two methods.

Due to its on-line nature, it only uses history in deciding to filter apoint. Hence the result will be rougher at the start of line and gettingbetter as it moves along.

Basic Operation.

The filter is started with a reset call to initialize the filter. Thereset is made using the first velocity measurement. The filter uses bothvelocity and position data. A cubic spine curve is created using thevelocity records and fitting the positions as good as possible to thiscurve.

Then the filter is reading a position record it is stored for laterprocessing.

When a velocity record is read a ‘knot’ is created. Any positions readbetween the previous and the present velocity records (in time) areadjusted to fit the curve.

History

The filter gain parameter, value 0 to 1, controls the influence ofDoppler-log data and history on the current point.

For the value 1 the Doppler-log data and history in the line have thegreater weight. Smaller values are only when there are more positionrecords than valid, velocity records.

Useful values will be in the range 0.9 to 1, e.g. 0.99.

Error Correction

The position and velocity records may be compared with predicted valuesusing previous data. Limits may be set when to reject data.

Resetting

If there are many erroneous data points there is a risk that the filterlooses track. The operator may reset the filter manually, i.e. kill itshistory (attempts are made to design an auto-reset).

Advantage: Disadvantage: It combines the best of Doppler and positions‘un-smooth’ at the start It is fast of line The overall result is smoothCan handle noisy Doppler data

1. A navigation processor interfaced to an acoustic processor and to aDGPS surface positioning equipment on board a vessel, the navigationprocessor being arranged to receive: i. position data from said DGPSsurface position equipment, ii. data from a fiber optic gyrocompass onboard an underwater system, the gyrocompass including heave, roll andpitch sensors, iii. depth data of the underwater system, iv. velocitydata of the underwater system based on data from a Doppler log unit onboard the underwater system; v. range and bearing data of the underwatersystem from said acoustic processor, in order to calculate attitudes andabsolute coordinates of the underwater system in a first coordinatesystem that is a common coordinate system.
 2. The navigation processoraccording to claim 1, wherein said common coordinate system isestablished between the navigation processor and a thruster controlprocessor that is interfaced with the navigation processor and arrangedto control thrusters on board the underwater system.
 3. The navigationprocessor according to claim 1, wherein said navigation processor isarranged to calculate an attitude of said vessel based on data from saidDGPS surface position equipment.
 4. The navigation processor accordingto claim 1, wherein said navigation processor is arranged to calculatefixed offsets of said vessel based on data from said DGPS surfaceposition equipment.
 5. The navigation processor according to claim 2,wherein the underwater system is provided with at least two thrusters,the navigation processor being arranged to control a driving force bysaid at least two thrusters in a thruster mean direction, the thrustermean direction and a first normal to this thruster mean directiondefining a second coordinate system.
 6. The navigation processoraccording to claim 5, wherein the navigation processor is arranged touse a third coordinate system defined relative to a system directionwhich is defined as the direction perpendicular to a lineinterconnecting the thrusters.
 7. The navigation processor according toclaim 5, wherein the navigation processor is arranged to calculate anerror in a path followed by the underwater system in terms of an errorvector that can be split into a first component parallel to the thrustermean direction and a second component normal to the thruster meandirection.
 8. The navigation processor according to claim 1, wherein thenavigation processor is arranged to receive the vessel's heading datafrom a vessel gyrocompass.
 9. The navigation processor according toclaim 2, wherein the navigation processor is arranged to perform acontrol loop to control orientation and position of said underwatersystem, and is arranged to receive a desired orientation and positionfrom an operator.
 10. The navigation processor according to claim 1,wherein the acoustic processor is arranged to receive data as to atleast one of local temperature and salinity of sea water surrounding theunderwater system and to at least one of temperature and salinityprofiles as a function of depth in the sea water, and to use these todetermine acoustic ray bending through the sea water and to correctcalculations of the position of the underwater system based on that. 11.The navigation processor according to claim 1, wherein the acousticprocessor is arranged to use a smoothing filter selected from a group offilters including a Kalman filter and a Salomonson filter.
 12. Aprocessor arrangement comprising a navigation processor and an acousticprocessor, said navigation processor being interfaced to said acousticprocessor and to a DGPS surface positioning equipment on board a vessel,the navigation processor being arranged to receive: i. position datafrom said DGPS surface position equipment, ii. data from a fiber opticgyrocompass on board an underwater system, the gyrocompass includingheave, roll and pitch sensors, iii. depth data of the underwater system,iv. velocity data of the underwater system based on data from a Dopplerlog unit on board the underwater system, v. range and bearing data ofthe underwater system from said acoustic processor, in order tocalculate attitudes and absolute coordinates of the underwater system ina first coordinate system that is a common coordinate system, where saidnavigation processor and said acoustic processor are implemented in oneor more computers.
 13. A measuring system comprising a navigationprocessor, an acoustic processor and an underwater system, saidnavigation processor being interfaced to said acoustic processor and toa DGPS surface positioning equipment on board a vessel, and theunderwater system comprising a fiber optic gyrocompass, a Doppler logunit and a depth sensor, the fiber optic gyrocompass including heave,roll and pitch sensors, the navigation processor being arranged toreceive: i. position data from said DGPS surface position equipment, ii.data from said fiber optic gyrocompass on board said underwater system,iii. depth data from the depth sensor of the underwater system, iv.velocity data of the underwater system based on data from the Dopplerlog unit on board the underwater system, v. range and bearing data ofthe underwater system from said acoustic processor, in order tocalculate attitudes and absolute coordinates of the underwater system ina first coordinate system that is a common coordinate system.
 14. Themeasuring system according to claim 13, wherein said depth sensor is adigiquartz depth sensor.
 15. The measuring system according to claim 13,wherein said common coordinate system is established between thenavigation processor and a thruster control processor that is interfacedwith the navigation processor and arranged to control thrusters on boardthe underwater system.
 16. The measuring system according to claim 15,wherein the underwater system is provided with at least two thrusters,the navigation processor being arranged to control a driving force bysaid at least two thrusters in a thruster mean direction, the thrustermean direction and a first normal to this thruster mean directiondefining a second coordinate system.
 17. The measuring system accordingto claim 16, wherein the navigation processor is arranged to use a thirdcoordinate system defined relative to the a system direction which isdefined as the direction perpendicular to a line interconnecting thethrusters.
 18. The measuring system according to claim 17, wherein thenavigation processor is arranged to calculate an error in a pathfollowed by the underwater system in terms of an error vector that canbe split into a first component parallel to the thruster mean directionand a second component normal to the thruster mean direction.
 19. Themeasuring system according to claim 13, wherein the navigation processoris arranged to receive vessel's heading data from a vessel gyrocompass.20. The measuring system according to claim 13, wherein the navigationprocessor is arranged to perform a control loop to control orientationand position of said underwater system, and is arranged to receive adesired orientation and position from an operator.
 21. The measuringsystem according to claim 13, wherein the acoustic processor is arrangedto receive data as to at least one of local temperature and salinity ofsea water surrounding the underwater system and to at least one oftemperature and salinity profiles as a function of depth in the seawater, and to use these to determine acoustic ray bending through thesea water and to correct calculations of the position of the underwatersystem based on that.
 22. The measuring system according to claim 21,wherein the acoustic processor is arranged to use a smoothing filterselected from a group of filters including a Kalman filter and aSalomonson filter.
 23. A method of measuring position and attitude of anunderwater system by a navigation processor on board a vessel, themethod comprising: i. receiving a position of the vessel based on datafrom a DGPS surface position equipment on board said vessel, ii.receiving data from a fiber optic gyrocompass on board said underwatersystem, the gyrocompass including heave, roll and pitch sensors, iii.receiving depth data of the underwater system, iv. receiving velocitydata of the underwater system based on data from a Doppler log unitonboard the underwater system, v. receiving range and bearing data ofthe underwater system, in order to calculate attitudes and absolutecoordinates of the underwater system in a first coordinate system thatis a common coordinate system.
 24. The method according to claim 23,wherein said common coordinate system is established between thenavigation processor and a thruster control processor that is interfacedwith the navigation processor and arranged to control thrusters on boardthe underwater system.